Combined immunodeficiency (CID) in foals will be utilized as a model of a similar disease in children. The objectives will include attempts to induce CID foal lymphocytes to differentiate in vitro to cells with identifiable T and B lymphocyte markers. Experiments will be made to determine if the epithelial remnant of CID foals is producing a thymus hormone capable of causing lymphocyte differentiation. Evaluation of the mixed lymphocyte culture response of affected foals and carrier parents will be done to determine if aberrant reactions occur. Enzymes of the purine salvage pathway, besides adenosine deaminase, will be measured in CID foal lymphocytes. Efforts to reconstitute lymphocyte function in affected foals will be made using fetal liver as a source of stem cells. Also, reconstitution will be tried using transfer factor and fetal thymus. Bibliographic references: McGuire, T.C., Banks, K.L., Evans, D.R., and Poppie, M.J. Agammaglobulinemia with B lymphocyte deficiency and normal T lymphocyte function. Fed. proc. 34: 841. 1975; McGuire, T.C. Banks, K.L., and Poppie, M.J. Combined immunodeficiency in horses: Characterization of the lymphocyte defect. Clin. Immunol. Immunopath. 3:555, 1975.